1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image on a recording medium.
2. Description of the Related Art
In an image forming apparatus of an electrophotographic system or electrostatic recording system, to suppress an image density difference due to a transfer memory, the following method is proposed. That is, a region (non-printing part) other than a part where a toner image is formed by an exposure unit is also exposed at an exposure amount weaker than an exposure amount for exposing the toner image formation part. For example, this technology is described in Japanese Patent Laid-Open No. 2008-8991. Hereinafter, the exposure on the region other than this toner image formation part will be referred to as background exposure.
In a DC contact charging system in which a DC voltage is applied to a charging roller for charging a photosensitive member, DC voltage of a high-voltage unit that applies the voltage to the charging roller may be fixed to a predetermined value in some cases to aim at reducing a size of the high-voltage unit. It is also proposed that the background exposure is performed at this time to cope with a change in a photosensitive member surface potential after the charging caused by a change in a film thickness of the photosensitive member or a change in a use environment (see Japanese Patent Laid-Open No. 2002-296853).
One of the methods of performing the background exposure is a technique for exposing an entire area of an image region at a weak light quantity (hereinafter, referred to as analog background exposure).
Another method is a technique for performing the background exposure on the non-printing part by setting an exposure time period per unit region to be shorter than an exposure time period for the toner image formation part (printing part). Hereinafter, the above-mentioned method will be referred to as digital background exposure (see Japanese Patent Laid-Open No. 8-194355). The digital background exposure is effective, for example, when the exposure cannot be performed at a weak light quantity because of a characteristic of a laser element used in the exposure unit.
In addition, a method of using a counting unit that is configured to count electric signals (video signals) received by a laser driver that controls the laser element provided in the exposure unit is proposed as a method of predicting a toner use amount. The counting unit samples a specified number of video signals in a previously set image region and counts the number of video signals that are ON. The toner use amount is predicted by calculating a printing rate of a printed image from a ratio of the sample number to the count value. Hereinafter, the above-descried method will be referred to as video-count toner use amount predicting detection. Since the signals received by the laser driver are actually directly counted, it is possible to accurately detect the toner use amount (see Japanese Patent No. 4822578).
However, when the above-described video-count toner use amount predicting detection is performed in the image forming apparatus to which the digital background exposure system is mounted, the following problems may occur in some cases.
The above-described counting unit measures any signals whatever the video signals received by the laser driver are. For that reason, the counting unit also measures the signals received by the laser driver at the time of the exposure of the non-printing part where the toner image is not formed. However, the toner is not consumed in the exposure on this non-printing part. Accordingly, when the toner use amount is to be predicted by the video-count toner use amount predicting detection, the video signals related to the non-printing part are unnecessarily measured, and the toner use amount may be detected to be higher than the actual toner use amount in some cases.
In view of the above, the present invention aims at obtaining a developer use amount based on electric signals for instructing exposure in an image forming apparatus that sets the exposure time period for the non-printing part per unit region to be shorter than the exposure time period for the printing part per unit region.